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Cracking the Code
on Sweat Rates
By
Gale Bernhardt
For Active.com
If you have been
doing triathlons for any amount of time, I suspect you do a good job of
sweating. Additionally, I'm sure you know that if you want to
successfully complete longer training sessions and races you must avoid
or delay dehydration caused by fluid losses from the body. Fluids are
lost through sweating, breathing and using the toilet.
Years ago the
advice was "drink, drink, drink," with experts assuming there was no
downside to consuming as much fluid as possible. Unfortunately,
consuming large amounts of water without electrolytes can lead to a
condition called hyponatremia. Hyponatremia, also known as low sodium
concentration or water intoxication, occurs due to prolonged sweating
coupled with the dilution of extracellular sodium caused by consuming
large amounts of fluid with low or no sodium.
Electrolytes
Sodium, chloride
and potassium are electrolytes, with sources divided on whether to
include magnesium in that group as well. Electrolytes remain dissolved
in the body's fluids as electrically charged particles called ions.
Electrolytes
modulate fluid exchanges between the body's fluid compartments and
promote the exchange of nutrients and waste products between cells and
the external fluid environment.
There is actually
an electrical gradient across cell membranes. The difference in the
electrical balance between the cell's interior and exterior facilitates
nerve-impulse transmission, stimulation and action of the muscles, and
proper gland functioning.
If you consume too
much water and not enough electrolytes, your body pulls electrolytes
from its cells in order to create the right balance for absorption. If
you consume too many electrolytes and not enough fluid, your body pulls
fluids from within to create the right balance for absorption.
The bottom line is
your body likes balance. Keeping your body in balance, or very close to
balanced, is part of your challenge as a sweaty endurance athlete.
Average
and Champion Sweat Rates
How much do we
sweat? An average person sweats between 0.8 to 1.4 liters (roughly 27.4
to 47.3 ounces) per hour during exercise. To help you with a visual, the
smaller bike water bottles typically hold 0.6 liters (20 ounces) of
fluid and the larger bottles hold 0.7 liters (24 ounces) of fluid.
The highest
recorded sweat rate for an athlete in an exercise situation is 3.7
liters (125 ounces) per hour, recorded by Alberto Salazar while
preparing for the 1984 Summer Olympics. The highest human sweat rate
recorded is 5 liters (169 ounces) per hour measured on a resting body
exposed to a hot environment. At rest, the skin blood flow was maximum
and not competing with exercising muscles.
How do you know if
you are an average sweaty person or a champion sweater? You need to do
some testing.
Your Sweat
Rate Test
The easiest way to
measure your sweat rate is to weigh yourself without clothes on before
exercising for one hour. After an hour of exercise, return home, strip
down and weigh yourself again. Assuming you did not use the toilet or
consume any fluids during exercise, your weight loss is your sweat rate.
For each kilogram of lost weight, you lost one liter of fluid. (For each
pound lost, you lost 15.4 ounces of fluid.)
If you drink any
fluids or use the rest room between the two weight samples, you'll need
to include both of these estimated weights in your calculations.
Add fluid consumed to the
amount of weight lost. Subtract
estimated bodily void weight from the total weight lost.
Be sure to record
the heat and humidity conditions in your sweat test. Repeat the test in
cool and hot conditions. Repeat the test for swimming, running and
cycling because sweat rates will vary for each sport and vary with
environmental conditions.
Now that you know
your sweat rates in each sport, you probably imagine that simply
drinking enough fluid will replace what you lose to sweat given the
environmental situation. If it were only that easy.
Fluid
Absorption Rates
Average fluid
absorption rates range from 0.8 to 1.2 liters per hour (27.4 to 40.6
ounces). Unfortunately, while the sweat-rate range and the
fluid-absorption ranges are close, some athletes sweat at higher rates
per hour than their fluid-absorption rate. In short races, the rate
discrepancy isn't much of a problem; however, for longer races the rate
difference can lead to dehydration and decreased performance.
Nausea and
vomiting can occur when athletes attempt to consume more fluids and fuel
than their bodies can handle. Even if you manage to hold down the extra
fluids, carrying beyond what your body needs is just unnecessary weight.
Champion
Fluid Absorption Rates
I was unable to
find any hard numbers for the maximum fluid-absorption rate documented
in a laboratory. Most scientific literature suggests there is a range of
absorption rates that varies from person to person. On the high end, I
have worked with three people that can consume in excess of 41 ounces
per hour.
On the low end,
some people develop intestinal fullness, nausea and vomiting when
ingesting fluid rates as low as 0.6 to 0.8 liters per hour.
Additionally,
scientists have found there is a specific water carrier in the body (the
water-channel protein, aquaporin) that influences fluid absorption.
There is speculation that people without aquaporin--or very low levels
of it--may have a reduced capacity to absorb fluid.
Case Study
During my coaching
career I've had the opportunity to work with three champion sweaters and
a couple of athletes I would consider low-volume sweaters.
The best data I
have for a champion sweater is a male triathlete that weighs around 185
pounds. He recently completed a 90-minute run at a very aerobic pace of
10 minutes per mile at a temperature range of 84 to 92 degrees
Fahrenheit with 70 percent humidity. He consumed fluid containing
electrolytes, water and some electrolyte tablets during the run. His
fluid consumption rate was 2.84 liters (96 ounces) per hour. He still
lost weight.
This particular
athlete has tested and retested himself to determine his sweat rate and
to determine how much fluid he can comfortably absorb. When he runs on a
hot day, even when he is acclimated to the heat, he routinely sweats at
a rate of 3.6 liters (121.7 ounces) per hour. He routinely consumes 2.84
liters (96 ounces) per hour. Even consuming fluids at a relatively high
rate, he loses 26 ounces of fluid or about 1.7 pounds per hour. Up to
about 3.7 pounds (roughly 2 percent of his body weight), the weight loss
has minimal affect on his performance.
Your
Numbers
If you are looking
to crack the code on your sweat rate and
related hydration rate, you need to start collecting data during
training and racing sessions. Know that your sweat rate is not a single
number, but changes depending on several factors not limited to your
fitness level, ambient temperature, humidity, clothing, exercise pace,
stress level and rest level. |
